1. Field of the Invention
The present invention is directed to a method for the operation of a magnetic resonance apparatus.
2. Description of the Prior Art
Magnetic resonance technology is a known technique for acquiring images of the inside of the body of a subject to be examined. In a magnetic resonance apparatus, rapidly switched gradient fields are superimposed on a static basic magnetic field. For triggering magnetic resonance signals, further, radio-frequency signals are emitted into the examination subject, the resulting magnetic resonance signals that are triggered being picked up, and image data sets and magnetic resonance images being produced on the basis thereof. The magnetic resonance signals are detected by a radio-frequency system, are demodulated in phase-sensitive fashion, and are converted into complex quantities by sampling and analog-to-digital conversion. These complex quantities are deposited as data points in a k-space dataset from which an image dataset, and thus a magnetic resonance image can be reconstructed with a multidimensional Fourier transformation.
Techniques referred to as functional imaging in medicine encompass all methods that utilize a repeated scanning of a structure of organs and tissues in order to image temporally changing processes such as physiological functions or pathological events. In the narrower sense, functional imaging in magnetic resonance technology is understood as measuring methods that make it possible to identify and image sensory stimuli and/or areolae in the nervous system stimulated by a motor, sensory or cognitive task, particularly the cerebral areolae of a patient. Acoustic and visual stimuli are examples of such sensory stimuli. In the simplest case, one of the sensory tasks comprises a defined movement, for example movement of the hand or of a finger.
The BOLD effect (Blood Oxygen Level Dependent) is the basis of functional magnetic resonance imaging. The BOLD effect is based on different magnetic properties of oxygenated and de-oxygenated hemoglobin in the blood. An intensified neural activity in the brain is assumed to be locally connected with an increased delivery of oxygenated blood, which causes a corresponding intensity boost at a corresponding location in a magnetic resonance image generated with a gradient echo sequence. The BOLD effect thereby occurs with a time delay of a few seconds relative to an event triggering the stimulation.
In functional magnetic resonance imaging, for example, three-dimensional image datasets of the brain are registered every two through four seconds, for example with an echo planar method. Echo planar methods thereby have the advantage that image dataset registration, at less than 100 ms required for an individual image dataset, is very fast. Image datasets with or without stimulation are thereby registered at different points in time. For forming a functional image, the image datasets registered with stimulation are subtracted from those without stimulation, i.e. the datasets are compared to one another for signal differences for identifying active brain areas.
In magnetic resonance technology, the homogeneity of the basic magnetic field is a decisive factor for the quality of the magnetic resonance images. Inhomogeneities of the basic magnetic field within an imaging volume of a magnetic resonance apparatus cause geometrical distortions of the magnetic resonance image that are proportional to the inhomogeneities. The homogeneity in sequences referred to as fast pulse sequences is especially important, for example in the echo planar method.
Recent developments in magnetic resonance apparatuses have been directed to creating devices with an examination space for the acceptance of the examination subject, for example a patient, that is accessible from all sides insofar as possible for the purpose of intra-operational interventions, and that is designed as spacious and open as possible because of patients having a tendency toward claustrophobia. The problem of distortions, particularly at the edges of the imaging volume and given an apparatus with a strong basic magnetic field, is intensified in these types of devices due to their very design.
Shim systems are utilized for improving the basic magnetic field homogeneity within the imaging volume. In a passive shim system, iron plates are attached in a suitable arrangement within the imaging volume. To that end, the basic magnetic field within the imaging volume is measured before the iron plates are attached. A calculating program determines the suitable number and arrangement of the iron plates from the measured values.
In an active shim system, shim coils that can be respectively charged with direct currents are utilized for homogenization of the basic magnetic field. Power pack devices that supply extremely constant and reproducibly adjustable direct currents are required for the operation of the shim coils. Among other things, an active shim system is used for fine correction when it is a matter of extremely high homogeneity is needed, for example in order to correct the field distortions caused by the examination subject at least partly placed in the imaging volume, particularly the field distortion within the examination subject.
As disclosed, for example, in German PS 195 11 791, the basic magnetic field within the imaging volume can be described with coefficients of a spherical function series expansion. This document also discloses correction of a linear inhomogeneity of the basic magnetic field, i.e. a field disturbance of the first order, by charging a gradient coil with an offset current. The offset current is a constant current that is superimposed on a gradient coil current that implements the gradient sequence. For compensating inhomogeneities of a higher order, a respective shim coils that each essentially compensate one of the coefficients are provided in conformity with the field disturbance to be compensated.
Due to the field-distorting effect of the examination subject, a shim setting procedure is implemented during the course of production of magnetic resonance images. Shim currents for the individual shim coils and offset currents for the gradient coils identified, for example once the region to be imaged has been positioned in the imaging volume. According to the aforementioned German PS 196 11 791, magnetic resonance signals of the examination subject are generated therefor with different echo times for forming two three-dimensionally spatially resolved raw datasets. The raw datasets are further-processed for determining corresponding shim and offset currents.
After the shim and offset currents have been set, non-compensated inhomogeneities of a higher order that continue to cause distortions in the magnetic resonance images are among the things that remain. The distortions in the phase-coding direction are dominant in the classic echo planar method. These distortions can be calculated out of the magnetic resonance image to a certain extent using a field map that describes inhomogeneities of the basic magnetic field. The field map is generated temporally after a setting of the active shim system, for example with a double echo gradient echo sequence. This technique for distortion-correction with a field map is explained in greater detail in, for example, the article by P. Jezzard et al., xe2x80x9cCorrection for Geometric Distortion in Echo Planar Images from B0 Field Variationsxe2x80x9d, Magnetic Resonance in Medicine 34, 1995, pages 65-73. The precision and accuracy of the distortion correction are thus in direct dependency on the degree of a local distortion.
An object of the invention is to provide an improved method for the operation of a magnetic resonance apparatus with which, among other things, the quality of the functional information can be taken into consideration.
This object is achieved in accordance with the invention in a method for the operation of a magnetic resonance apparatus wherein an anatomical image of a region of an examination subject to be imaged is generated with a prescribable resolution and quality, a functional image of the region to be imaged is generated, the functional image is superimposed on the anatomical image, and at least one area of the region to be imaged wherein the functional image contains unreliable information is correspondingly identified in the superimposition.
As a result thereof, areas with questionable or suspicious functional information are emphasized for a diagnostician, so that a local quality of the functional information can be estimated and misinterpretations and/or misdiagnoses connected therewith can be reduced.